28th Symposium of AER on VVER Reactor Physics and Reactor Safety (2018, Olomouc, Czechia)
Fuel management issues
Pavel Mikoláš, Jan Vimpel (ŠKODA JS)
In order to increase the efficiency of the fuel utilization at Dukovany NPP, the idea of a change in the structure of the FA, whose basic characteristic is shroud removal and its replacement with structure called “Karkaz”, was adopted. Against the original proposal of the Russian fuel supplier, this design is characterized, among other things, by the fact that the “angles” at the corners of the FA are longer and “overlap” the outer row of fuel rods, with the exception of one “central”. This solution was designed to reduce transverse flow between the FAs and thereby “de facto” maintain the FA property as an isolated set. In addition, the structure of the angles is significantly thinner than the original shroud, and this allows the increase of the pitch between the fuel rods up to 12.6 mm (instead of standard 12.3 mm spacing) to reduce parasitic absorption in this component and improve neutron moderation, thereby improving the neutron-physical characteristics of such FA, resulting in more energy being obtained from a given amount of the fissionable material. The fuel pin is assumed to be unchanged with respect to pin in the FA type Gd-2M+. (Note: The control assemblies remain shroud-type, but there is a change in fuel pin with Gd2O3, such that this fuel pin will also have a tablet of 7.8/0.0 mm.)
Optimization of PK3+ FAs type of different average enrichment was performed in order to find such a radial enrichment profile in FA with the aim to minimize the non-uniform energy generation in FA (during burning).
In addition, optimization was argued that a better (lesser) energy equalization could be achieved by using a change in the location of fuel pin with Gd2O3 burnable absorber – from the 2nd row to the 3rd row of rods (pins) from the edge of the fuel assembly on the fuel assembly diagonal.
The aim of this study is to achieve a full quadruplicate cycle, each of 15 months (approx. 450 days) at 1475 MWt nominal power.
Preliminary results indicate that fuel assemblies combination PK-3+ and Gd-2M+ does not show any unusual phenomena from the point of view of reactor physics. On the other hand, due to the loading of fuel assemblies in combination of 60+12 and 66+6 pieces after several cycles, 1 FA with a burnout in the range of 40-45 000 MWd/tU remains. This is particularly problematic for batch design, because Athena optimization algorithm places this FA around the core center where frequent fresh or 2-year assemblies are often in positions 3 and 12. As a result, this FA has a high burnout increase during one campaign. This increase may be as high as the limit for burnout of the fuel pin, which is set on 62000 MWd/tU, is exceeded.
This strategy is based on the B1C33 campaign implemented at the Dukovany NPP, which is designed to be 395 FPD days. Already the first cycle, which is loaded with 60 fresh PK-3+ FAs and 12 Gd-2M++ CAs, the reached length at EOR is 424 FPD, which means stretchout 26 effective days. On average, the transition cycles stretchout length is 23.5 effective days. For steady cycles this average value is 19.2 effective days.